BEIJING — China’s security services have detained several close relatives of four U.S.-based reporters working for Radio Free Asia in an apparent attempt to intimidate or punish them for their coverage of the Muslim-majority Xinjiang region, the news organization said Wednesday.

Tens of thousands of Muslim ethnic Uighurs have been detained in “political education centers” by Chinese authorities in the western province of Xinjiang in recent months, according to Human Rights Watch. The crackdown is portrayed as a “strike hard” campaign against terrorists and separatists but effectively means that those who express their religious or cultural identity are targeted, Human Rights Watch said.

“We’re very concerned about the well-being and safety of our journalists’ family members, especially those in need of medical treatment,” said Rohit Mahajan, director of public affairs at Radio Free Asia in Washington.

“We’re also particularly concerned about the use of detentions as a tactic by Chinese authorities to silence and intimidate independent media, as well as to inhibit RFA’s mission of bringing free press to closed societies.”

Among those who have been detained or have disappeared are several close relatives of Shohret Hoshur, Gulchehra Hoja, Mamatjan Juma and Kurban Niyaz — four ethnic Uighur journalists with Radio Free Asia in Washington. The first three are U.S. citizens, while Niyaz is a green-card holder.

Their reporting for the U.S. government-funded news organization has offered one of the only independent sources of information about the crackdown in the province.

All three of Hoshur’s brothers were jailed in Xinjiang in 2014, but two were released in December 2015 after the U.S. government protested. The third, Tudaxun, was sentenced to a five-year jail term in 2015 on charges of endangering state security and remains in prison.

Hoshur said the other two brothers were detained again in September and taken to the Loving Kindness School, a political reeducation center in the city of Horgos. Hoshur said a source told him that around 3,000 people have been detained there.

Hoshur said Chinese authorities have contacted family members living in Xinjiang, urging them to ask him to stop calling and reporting on events in the region.

In a separate statement posted online last week, Hoja said her brother, 43-year-old Kaisar Keyum, was taken away by police in October and his whereabouts are unknown. Since late January, she has also lost all contact with her parents, who are both in their 70s and in poor health.

“My father is paralyzed on one side and needs a constant care. My mother has recently had a surgery on her feet and is very weak,” she said in the statement. “I need to know where they are and that they are OK. I need to be able to speak to them. They have not committed any crime.”

Shortly after calling her aunt earlier this month, Hoja said, she received a call from a friend in West Virginia whose mother lives in Urumqi, Xinjiang’s capital. Her friend said that around 20 of Hoja’s relatives have been arrested by the Chinese police because of her reporting.

When her brother was detained, police told Hoja’s mother that her employment with RFA was the reason for his detention, the news outlet said. It said Hoja has heard that her relatives may have been detained for being in communication with her through a WeChat messaging group.

Juma, deputy director of RFA’s Uyghur Service, reported that his brothers Ahmetjan and Abduqadir Juma were detained in May 2017. Ahmetan’s whereabouts are unknown, while Abduqadir has been taken to a prison in Urumqi. He suffers from heart and health issues that require medical care, but his sister has been denied access to him.

“The family is deeply concerned about his health and well-being while being held in a prison known for its inhumane conditions,” RFA said.

RFA Uyghur broadcaster Niyaz’s youngest brother, Hasanjan, was arrested last May and soon afterward sentenced to six years in jail for “holding ethnic hatred.”

Human rights groups say China represses the rights, culture and freedom of worship for Uighur Muslims. Xinjiang has been home to long-running separatist unrest, and several violent attacks have occurred there in recent years, blamed by the authorities on Islamist extremism.

In a report issued Tuesday, Human Rights Watch described how a system of predictive policing, involving constant mass surveillance and big data analysis, was being deployed to bolster the crackdown in Xinjiang.

The police gather data from all-pervasive security cameras, some of which have facial recognition or infrared capabilities, the report said. “WiFi sniffers” monitor smartphones and computers, while car license plate and identity card numbers are gathered at the region’s countless security checkpoints, all cross-checked against health, banking and legal records, it said.

Police officers, Communist Party cadres and government workers also visit homes to gather data on families, their “ideological situation” and their relationships with neighbors. One interviewee said even owning a large number of books could arouse suspicion, unless one worked as a teacher. Data is also gathered on frequency of prayer and visits abroad.

Constant surveillance and harassment have made it extremely difficult for foreign reporters based in China to cover the crackdown in Xinjiang effectively, with locals too scared to talk to reporters and security officials obstructing or detaining several journalists who have ventured there.

RFA said it has been in contact with the State Department over the detentions, but China’s Ministry of Foreign Affairs declined to say whether it has received any communications from the U.S. government.

Foreign ministry spokesman Lu Kang said that, on the question of reporting, “we welcome and support foreign media to report on China in an objective and fair way.”

RFA was set up by Congress in 1994 to broadcast news that would otherwise not be reported in Asian countries where governments do not allow a free press, and it continues to be funded by an annual grant from the U.S. government’s Broadcasting Board of Governors.

Hoshur said China might be using voice recognition technology to intercept his phone calls to gather information from Xinjiang. Almost all of them are cut off in under a minute, he said.

The State Department said in a statement Wednesday, “We urge the Chinese government to cease policies that unduly restrict the exercise of freedom of religion or that otherwise deny individuals their ability to enjoy their human right.

“We call on China to release all prisoners of conscience, and to respect the human rights and fundamental freedoms of all its citizens.”